Phase modulation of coherent light allows high-efficiency holographic image projection and beam steering; the latter applications include Optically Transparent Switches for optical networks, for Add-Drop Multiplexers for Wavelength Division Multiplexing (WDM) Telecomms, and TV multicast. Incident illumination from the incoming fibres can be randomly or variably polarised or polarisation multiplexed, and the overall rotation of polarisation of the original signal into the fibre may vary with time or day, temperature, mechanical stress on the fibre, etc. To deflect and route these signals without unacceptable losses or continuous adjustment polarisation insensitive methods are needed.
One method of routing signals uses a Spatial Light Modulator (SLM) which instead of modulating luminance modulates the phase of the reflected light in the relative range 0 to 2π, and presents blazed gratings or suitable holograms to steer the incoming signals to different output ports, e.g. using a Liquid-Crystal-on-Silicon (LCOS) backplane to display the phase hologram. The Liquid Crystal (LC) material may be a nematic material, generating an analogue blazed grating (in which case only a single linearly polarised component of the signal is modulated), or a suitable ferroelectric LC which can be polarisation insensitive but has the disadvantage that only binary phase gratings or holograms can be formed, resulting in an extra 3 dB routing loss. Background prior art can be found in U.S. Pat. No. 5,319,492 and JP2002/357802A.
In general it is desirable to suppress unwanted reflections in such devices; for some applications such as telecoms this is particularly important to suppress unwanted crosstalk. Background prior art relating to anti-reflection structures can be found in: U.S. Pat. No. 7,542,197; GB2,430,048A; US2012/0057235; and WO2012/123713.
Random polarisation can be accommodated by splitting the incoming signal into two orthogonal polarisation streams, routing each separately using suitably oriented nematic SLMs, and recombining them (with additional losses and a requirement for careful path length balancing), or by using a technique such as an internal quarter-wave plate.
Light travelling through a medium can be disrupted by variations in refractive index—e.g. in the atmosphere due to turbulence giving pressure changes or convection caused by temperature changes. For any object viewed in the far field, this alters the shape of the plane wave across the entry optics, and limits the resolution of the optics below the theoretical limit. E.g. for an astronomical telescope this gives a blurred jittering image of a star which should be a point source. For planetary or surveillance images this also gives instantaneous spatial distortion.
Adaptive optics can partially compensate for the disturbance, usually by using a deformable mirror with an array of electro-mechanical actuators. These can correct the wavefront deformation of a few microns and partially restore the wavefront.